A received radio signal has often been corrupted by noise and intersymbol interference caused by e.g. multipath propagation. The multipath propagation may be e.g. of a Rician type where there is one (direct) path with a strong received signal, and other (reflected) paths with smaller signal strength. The Rician type propagation is usual in satellite systems. Another type of multipath propagation is a Rayleigh fading channel, where the strength of the received signals from different paths are of the similar magnitude. The Rayleigh fading channels are typical in cellular communications systems with fixed base stations.
A functional block known as the signal equalizer is often used in TDMA (Time Divisional Multiple Access) receivers for recovering transmitted data from a received signal. In CDMA receivers a rake receiver is used performing this function. Typical radio receivers that use signal equalizers and Rake receivers are mobile stations and base stations of cellular radio systems. A signal equalizer needs to know the impulse response of the radio channel for the equalization to be successful. The conventional way of performing channel estimation and signal equalization is to generate an estimate of the radio channel's impulse response (also known as the channel estimate for short), and to equalize the received transmission blocks by using the achieved equalization data. A rake receiver typically comprises a channel estimator for each rake finger for estimating a complex channel multiplier for each signal path to be corrected. An example of a prior art solution for providing one rake finger in a WCDMA (Wideband Code Division Multiple Access) rake receiver is shown in FIG. 1.
FIG. 1 illustrates a prior art receiver arrangement for receiving a CDMA signal. An analog oscillating signal on a radio frequency is received through an antenna 102, downconverted onto a complex baseband frequency signal in a radio receiver 104 and converted into a series of digital samples in an AID converter 106. In the present CDMA receiver the despreading of the signal is performed by first leading the samples to a multiplier 110 for multiplying the received samples by a complex conjugate of the long code (also called a “scrambling code” in WCDMA). The signal from the multiplier 110 is led to another multiplier 120 for multiplying the signal with a short code. The achieved, despread signal is then integrated in block 122.
The signal from the multiplier 110 is also led to an integrator 112 and further to a channel estimator 114. The channel estimator estimates the complex channel coefficient of the radiochannel using pilot signal information (or a training sequence in a TDMA receiver), and provides the channel estimate for removing the channel. The despread signal from the integrator 122 is then multiplied by the complex conjugate of the channel estimator output in order to remove the phase shift caused by the channel. The output includes the recovered data (the so-called hard decision output) and it may include reliability information (soft decision output) associated with the recovered data.
The output is further transformed into a real signal in block 132. The channel decoding operation may comprise additional operations like de-interleaving, and the reconstructed information symbols may be conveyed further e.g. to an audio or video decoder, to a data storage device or to some control circuitry.
An article [1] “A Novel Pilot Symbol Assisted Coherent Detection Scheme for Rician Fading Channels” by T. Asahara, T. Kojima and M. Miyake, WPMC ′98, pp. 236-239, 1998, presents an advanced prior art method for equalizing a radio channel. The method is developed for receivers in a satellite communication system, where the channels are of Rician type.
With the prior art channel equalization of [1] and FIG. 1 it is possible to compensate phase shifts that are present in the received signals due to the multipath radio channel characteristics. However, the prior art arrangements of [1] and FIG. 1 are not able to sufficiently correct frequency offsets that may exist. And even if the prior arrangements are able to correct some of the frequency offset, the frequency offset still makes the channel estimates less accurate which causes losses in performance.
There are two main causes for frequency errors or “frequency offsets”. The first one is a frequency offset in the receiver oscillator which is used for downconverting the received RF signal. This means that there is a frequency offset between receiver oscillator frequency and the carrier frequency of the base station. This offset, for example, degrades the performance of the channel estimator.
Another cause for a rotation of the signal constellation is a so-called doppler effect. This means that the length of the radio signal path between the mobile station and the base station changes when the mobile station moves. This causes a doppler spectrum in a received signal. Since a mobile station adjusts its transmission frequency according to the received carrier there will be a frequency error due to the doppler effect in the received signal. Especially with high mobile speeds the residual doppler effect can be large and this will also degrade the performance of a channel estimator.
In mobile station receivers the frequency offset may be detected and frequency of the local oscillator (RF oscillator or IF oscillator) signal can be controlled to remove the offset. However, in the prior art receivers there is no information available on the exact amount of the frequency offset. Another problem is that the resolution of the oscillator frequency adjustment is usually too coarse for adequately compensating the frequency offset. In base station receivers it is not possible to adjust the local oscillator frequency, because the local oscillators are common for several channels, and the amount of doppler effect is usually different for signals that are received from different mobile stations. It would also be possible to use adaptive channel equalizers for compensating the frequency offset, but these kind of equalizers require large amounts of memory and processing capacity, and therefore they would substantially increase the manufacturing costs of the receivers.